As previously discussed, structures which contain and distribute cooling air to the vane segment shrouds are typically affixed to the surface of the shrouds opposite those surfaces exposed to the hot gas flowing through the turbine section. These structures are referred to as "external" cooling air structures to distinguish them from structures for distributing cooling air which are formed inside the airfoil portions of the vane segments. During operation, the shrouds get very hot as a result of the flow of the hot gas over them. The structures, however, have cooling air flowing over them and hence do not get nearly as hot as the shrouds. As a result, severe thermal stresses are induced in the structures due to the differential thermal expansion between the shroud and the structure.
According to the prior art, the thermal stresses were reduced by forming the structures from thin plates, thereby making them as flexible as possible. However, a minimum amount of strength and stiffness is necessary to ensure that the structures can withstand the pressure of the cooling air inside them. As a result of this trade off between strength and flexibility, the prior art approach has yielded less than optimum results.
Accordingly, it would be desirable to provide an apparatus and method for minimizing the differential thermal expansion between the shrouds and the external cooling air structures attached to them.
In the past, certain components exposed to hot gas flow in the combustion section of a gas turbine, such as combustors or transition ducts, have been formed from laminates. The laminates themselves are formed by joining two thin plates in a sandwich-like fashion. Typically, one or more internal passageways, in a straight through or serpentine arrangement, are formed between the layers of the laminate. Cooling air flows through these internal passageways and cools the component. According to the present invention, novel use is made of such laminates by forming vane segment external cooling air structures from them. Rather than using the internal passageways for cooling purposes, hot gas flowing over the shrouds is directed through the internal passageways. The flow of hot gas heats the structures, thereby minimizing the differential thermal expansion between them and the shrouds to which they are attached.